Hydrogeochemical processes and influence of seawater intrusion in coastal aquifers south of Chennai, Tamil Nadu, India.

Seawater intrusion promotes the salinity of groundwater, and it poses a great environmental impact on a global scale. The present study was carried out to determine the hydrogeochemical processes and influence of seawater intrusion in the coastal aquifers using geophysical, geochemical, and stable isotope techniques. The true resistivity value ranges from 0.5 to 8008.5 Ω-m which has been measured using vertical electrical sounding (VES) based on the Schlumberger method. About 33 groundwater samples were collected during post-monsoon (POM) (January 2012) and pre-monsoon (PRM) (June 2012) seasons from open and bore wells and were analyzed for major ions and stable isotopes. EC, Na+, and Cl- were high in groundwater of wells near salt pan, the Buckingham Canal, and backwater regions. Around 45% of the groundwater of this study area is of Na+-Cl- type due to salinisation. Reverse ion exchange and silicate weathering are the dominant processes controlling the geochemistry of groundwater. Saturation indexes (SI) of halite (SIhalite) and gypsum (SIgypsum) versus sulfate show an increasing trend line from > 0 to < 0, which implies higher dissolution of minerals and hints increasing salinization during both seasons. The value of Na+/Cl- ranges between 0.7 and 2.4 (POM) and from 0.6 to 2.8 (PRM). The molar ratio suggested that around 25% of the groundwater samples are with values similar to those of seawater. Further, the groundwater is also affected by saline backwater, salt pan activities, and Buckingham Canal. Some locations are also are affected by anthropogenic, agricultural activities and geochemical processes. Heavy stable isotopes were found to be dominant in the coastal region due to seawater intrusion. Stable isotopes of δ18O range from - 5.6 to - 2.9‰ during both periods. About 201 km2 of this area is affected by salinization. It is necessary to reduce pumping and plan for physical barriers to create freshwater ridges for controling the seawater intrusion.